AGE OF GYPSUM OF CENTRAL IOWA 733 



Climatic conditions in both hemispheres, therefore, seem to 

 have been favorable for deposits of gypsum during the Permian, 

 whereas, if the Iowa gypsum were referred to the Cretaceous, it 

 would be the only gypsum deposit of economic importance in 

 Europe or America assigned to this period of geological histor}-. 

 The gypsum may therefore be reasonably regarded as Permian, 

 though the possibility of its being Triassic cannot be denied. 



ORIGIN OF THE GYPSUM, 



Gypsum deposits are generally ascribed to two causes: (i) 

 the transformation of deposits already formed by various chem- 

 ical reactions, and to reactions between the salts in solution, or 

 (2) to precipitation from sea water, due primarily to concentra- 

 tion by evaporation. 



I. The most frequent transformation of deposits already 

 formed is the change of limestone (CaCOg) into gypsum (CaSO^ 

 -f 2 HgO), through the agency of sulphuric acid, according to 

 the equation H^SO^ + CaCOg == CaSO^ + COg + H3O. The 

 sulphuric acid may be generated by the oxidation of the sul- 

 phuretted hydrogen of sulphur springs or of volcanoes, or by the 

 action of water on some sulphide ore like pyrites. The deposits 

 which Dana attributes to the action of sulphuric acid generated 

 from the sulphuretted hydrogen given off by sulphur springs in 

 New York'^ are quite extensive. In certain instances the gypsum 

 occurs in masses with irregular outline in limestone, and layers 

 of shale in the limestone pass unaltered through the gypsum. In 

 view of this evidence, the gypsum must be regarded as derived 

 from the limestone. Deposits of this sort are exceptional, how- 

 ever, and it is probable that most of the gypsum of New York 

 had a different origin. 



Insignificant gypsum deposits occur about the fumaroles of 

 craters and lava streams in Hawaii where sulphurous acid SO^ is 

 converted into sulphuric, and attacks rocks which contain lime. 

 The frequent occurrence of small amounts of gypsum with 

 hematite in the upper part of ore veins may be accounted for by 

 the following reaction;^ Fe^Og (SO^^g -|- 2 CaCO., =^ 2 CaSO^ 



' Dana, Manual of Geology, 4th ed., p. 554. 'Beck, Erzlagerstattenlehre, p. 393. 



